Kinematic chains are used in a variety of applications to provide elongate linkage structures. Examples of structures which employ such kinematic chains include, but are not limited to, robotically controlled instruments and/or minimally invasive surgical instruments. Minimally invasive surgical instruments can be either manually controlled or robotically controlled. Kinematic chains can include, as the entire chain or one or more parts thereof, structures analogous to the human skeleton and formed of a series of interconnected links (analogous to vertebrae). The links can be coupled to one another via joints to permit articulation of adjacent links relative to each other in one or more degrees of freedom (DOF). Through this articulation, these jointed link structures can likewise bend in one or more degrees of freedom (e.g., including pitch and/or yaw bending) to be placed in various curved shapes, analogous to a snake-like movement. Force transmission elements, such as, for example, tension members (e.g., cables, wires, and/or push rods) and/or compression members (e.g., push rods, Bowden cables), may interconnect the series of links to exert actuation forces that tend to articulate the links relative to one another to provide a desired overall bending and curved shape of the jointed linked structure.
Minimally invasive surgical techniques, whether performed manually or via robotically assisted systems, generally attempt to perform surgical procedures while minimizing damage to healthy tissue. One particular technique for achieving this goal employs surgical devices having bendable, steerable arms, wrists or the like that are formed of jointed link structures which are able to reach a target work site inside a patient by at least partially following a natural lumen, such as, for example, the digestive tract, blood-carrying lumens, bronchi, or other lumens, of the patient. Following a natural lumen, for example, can allow a surgeon to operate at a work site while making fewer and/or smaller incisions through healthy tissue, although an incision may be needed at locations where the surgical device enters or leaves a natural lumen. Surgical devices that are able to follow a natural lumen or other tortuous path must therefore be flexible so as to permit bending (e.g., curving) and also steerable so as to permit control over such bending. Various surgical applications, particularly those associated with small lumens and/or lumens having tight bends or bifurcations, for example, as encountered in pulmonary and/or neurological applications among others, may require portions of an elongate surgical device to exhibit a relatively small radius of curvature when bending, particularly in order to reach the ultimate work site. In general, depending on the application, various curvatures and degree of curvatures may be desired along a length of the bendable, steerable device.
In some cases, jointed link structures can be disposed at a distal end portion of an elongate surgical instrument (i.e., an end portion of the instrument closest to the work site when the instrument is in use) where controlled steering, bending with small radii of curvature, and/or large and/or multiple DOF motion can be beneficial. Accordingly, a wrist that is formed by a jointed link structure can be provided toward the distal end of an elongate surgical device to support an end effector (e.g., a tool such as a clamp, blade, scissors, cautery member, etc., and combinations thereof). Entry guides (e.g., guide tubes) and/or other similar endoscopic structures that facilitate insertion and removal of surgical instruments and tools, and that facilitate reaching a target work site of a minimally invasive surgical procedure, also can be formed of jointed link structures to provide desired flexibility and bending to enable those devices to traverse and be steered along tortuous paths, such as, for example, body lumens. Entry guides generally contain one or more channels through which other surgical instruments can be inserted and removed. Using an entry guide can also allow instruments to be changed without requiring a delicate steering procedure each time a different instrument is needed.
Aside from the ability to traverse narrow and tortuous paths, the overall size of minimally invasive surgical instruments may pose constraints on the design of such instruments. In general, it is desirable for the overall size, including the outer lateral dimensions (e.g., diameter), of such instruments to be relatively small to fit within narrow lumens and other passages. In some cases, therefore, it is desirable to select the number and placement of force transmission elements so as to reduce the overall size of the jointed link structures. For example, the number and placement of force transmission elements that interconnect a series of articulably coupled links to provide actuation forces to control bending of the structure may be such that the one or more force transmission elements pass through one or more links without directly attaching and terminating at such links. For example, one or more force transmission elements may pass through one or more proximal links (“intermediate” links) of a series of links and ultimately terminate and directly attach to a distal link (“termination” link) of the series, with the intermediate links of the series not having any force transmission elements terminating and directly attaching thereto. Accordingly, the bending and steering of a plurality of joints (or link pairs) in a series is actuated through a single force transmission element (or single set of force transmission elements in the case of multiple bend directions and or DOFs) without each joint or link pair including a termination link and thus being capable of individual direct bending by actuation of a force transmission element directly attached to such a link pair. Such a configuration is sometimes referred to as “underconstrained.” In other words, the steering and bending of multiple link pairs is actuated by a single force transmission element or single set of force transmission elements that is attached to and terminates at a link of one of the link pairs in a series of such link pairs. Such “underconstrained” structures, however, can pose challenges in attempting to controllably steer and bend, thereby resulting in unpredictable and/or uncontrollable movement (articulation) of the links.
Controls for such jointed link structures, including underconstrained structures, are sometimes based on a so-called “equal angle assumption,” wherein it is assumed that a series of link pairs, for example, that includes intermediate links bounded by a termination link, are bent at roughly the same angle about their respective joints in response to actuation of the one or more force transmission elements directly attached to the termination link. Depending on various forces that may be acting on the links, such as, for example, joint friction, and resistances due to skin and/or payload of the structure, the equal angle assumption may not accurately reflect the actual orientation of link pairs. Moreover, in some cases, particularly when such structures are used in very small lumens and other passages, it may be undesirable to provide a series of interconnected link pairs that all bend at about the same angle since doing so can result in a larger overall bend radius of curvature of the series of link pairs. For a further explanation of jointed link systems that incorporate control systems based on an equal angle assumption, reference is made to U.S. Pat. No. 6,817,974 B2, (filed Jun. 28, 2002; entitled “Surgical Tool Having Positively Positionable Tendon-Actuated Multi-Disk Wrist Joint”), which is incorporated by reference herein in its entirety.
It may be desirable, therefore, to provide a jointed link structure which can be more controllably bent in order to perform a wide range of minimally invasive surgical procedures, including those which require traversal of small, tortuous paths to a work site. It also may be desirable to provide a jointed link structure that can be more controllably bent while the overall dimensions (e.g., including diameter) of the structure is kept relatively small. Moreover, it may be desirable to provide jointed link structures that are relatively robust in design and relatively inexpensive to manufacture, and that can achieve controlled bending through the use of relatively simple mechanical designs.